Potentially, we could see some celestial dragon characters. They might be ancient and powerful beings. Some could be friendly towards Natsu, seeing him as a new hope for their kind. Others might be more hostile, seeing him as an intruder or a threat to their traditional ways. There could also be other mages from different guilds who are either curious or envious of Natsu's situation with the celestial dragons.
We might also encounter some villains who are specifically after the power that Natsu has gained from his connection to the celestial dragons. These villains could be from dark guilds or have their own evil motives. And there could be some mysterious characters who seem to know more about Natsu's situation than he does. They could be mentors or just shadowy figures who manipulate the events in the background.
Happy, Natsu's flying cat companion, would definitely be a main character. He would be there to provide comic relief as always, but also be a source of emotional support for Natsu during his transformation into a dragon god. Erza might also be a prominent character. With her strong leadership skills and powerful magic, she could be guiding the guild during the times when Natsu is still coming to terms with his powers and the changes it brings to the guild.
In addition to Natsu, Wendy could be a main character in the 'natsu is a dragon god fanfic'. Since she also has dragon - related powers, she could have a special connection with Natsu in this new situation. Maybe she helps him understand more about his dragon god powers. And Makarov, as the guild master of Fairy Tail, would also be a significant character. He would be concerned about how Natsu's new status affects the guild and the magical world as a whole.
Gray might show up too. He could start as a bit of a rival, not believing that Natsu can really achieve great things with this Dragon God training. But as the story progresses, he could become more of a supporter. And then there's Lucy. She might be worried about Natsu during the training, and also use her celestial spirit magic to help Natsu in some unexpected ways during his training, like summoning spirits that can provide unique challenges for Natsu to overcome.
Another potential character could be a dragon oracle. This oracle could have the ability to see Natsu's true nature and destiny. They could either help Natsu understand his powers and purpose or be a bit of a trickster, making Natsu's journey to discover himself even more difficult. And of course, the other members of Fairy Tail would be there to support or sometimes misunderstand Natsu as he deals with his dragon - born identity.
There could be a female dragon named Ignis who is very fiery and passionate, always trying to be the closest to Natsu. Another could be a more gentle dragon named Aqua, who has water - based powers and uses her calm nature to balance out the more intense dragons in the harem.
One of the main characters could be Happy, Natsu's loyal flying cat companion. Happy would likely be by Natsu's side during his retirement, still providing his comical relief and support. Another character could be Lucy. She might be concerned about Natsu's retirement decision and try to understand his reasons, or she could also be retiring from her own magical adventures and join Natsu in his new peaceful life.
There could be new human characters as well. Maybe some mages from other guilds who are curious about Natsu's new status. They could either be allies or potential threats. And don't forget about the celestial spirits. Since Lucy is Natsu's friend, the celestial spirits might get involved in some way. They could offer their unique powers or knowledge to help Natsu in his new role as the dragon king.
Lucy could be a main character. Since she's often by Natsu's side, she would likely be deeply involved in his recovery. She could use her celestial spirit magic to try and protect him while he's injured or search for a way to heal him.
Igneel could also play a role. Since Igneel was Natsu's adoptive dragon father, he could appear in Natsu's mind during the transformation, guiding him and giving him advice on how to handle his new dragon powers. And there could be other dragons in the story as well. They might be either friendly or hostile towards Natsu depending on their nature.
There could be some new dragon characters created specifically for this fanfiction. These dragons might have unique abilities and characteristics that are different from the known dragons in the original Fairy Tail series. And perhaps there are some wise old wizards or spirits in the fanfiction who guide Natsu in between his dragon - training sessions.